This invention relates generally to rheology-modified thermoplastic elastomer (TPE) compositions that comprise an elastomeric ethylene/alpha (xcex1)-olefin (EAO) polymer or EAO polymer blend and a high melting propylene polymer, wherein both components are peroxide-modified and to the preparation of the compositions, use of such compositions in processes such as calendaring and thermoforming to make articles of manufacture and the resulting articles of manufacture. This invention particularly relates to such compositions wherein the rheology modification is induced by a combination comprising an organic peroxide and a free radical coagent, methods for preparing the compositions, such as by modifying a physical blend of the components, and use of such compositions in calendaring operations and thermoforming applications.
Heck et al. describe rheology modified TPE compositions in WO 98/32795. The rheology modification can be induced by various means including peroxides and radiation. The compositions of Heck et al. are said to exhibit a combination of four properties: shear thinning index (STI), melt strength (MS), solidification temperature (ST) and upper service temperature (UST). While these compositions are useful in applications such as automotive parts and boot shafts, improved compositions are needed for calendaring operations and thermoforming applications.
Compositions having a high melt toughness are desired in calendaring operations. Melt toughness, as used herein, is the product of the melt strength and the melt extensibility. In many instances, the calendar rolls are fed with a composition in the form of a molten rod. This molten composition must be able to spread across the calendar rolls. The Heck et al. compositions only spread partially across the calendar rolls.
Compositions having a high melt toughness also are preferred for thermoforming applications. In addition, tensile properties of the compositions at elevated temperatures are important for these applications. For example, one method of manufacturing instrument panel skin material is to either calender or extrude embossed sheeting. The sheeting is then vacuum thermoformed to the contour of the instrument panel. One method to determine compound thermoformability is by evaluating its elevated stress-strain behavior. Often, flexible polypropylene thermoplastic (TPO) sheets are thermoformed at temperatures below the melting point of the polypropylene phase. Although the thermoforming process is one of biaxial extension, tensile tests at the thermoforming temperatures can be used to compare thermoforming and grain retention behavior. The peaks and valleys of the embossed grain are areas of greater and lesser thickness and a look at the grain shows that the valleys are narrower and less glossy than the peak areas. When a skin is thermoformed, the thinner areas will be subject to greater stress and the greater applied stress in these areas concentrates the elongation in the thinner valley areas. These areas elongate preferentially and the attractive xe2x80x9cnarrow valley, broad peakxe2x80x9d appearance is lost, called xe2x80x9cgrain washoutxe2x80x9dxe2x80x94unless the material can be designed to elongate more evenly. Strain hardening is the property by which areas of material which have already been strained become stiffer, transferring subsequent elongation into areas which are as yet unstrained. Strain hardening thus allows a thermoformed skin to exhibit more evenly distributed elongation and minimized grain washout.
One classic way to examine the strain hardening behavior of a material is the Considxc3xa8re construction, by which the true stress, defined as the force across the instantaneous cross sectional area is plotted against the draw ratio. Regular stress-strain graphs calculate the strain using the initial cross-sectional area, but the cross sectional area diminishes as the sample is strained. The Considxc3xa8re construction is often used to evaluate cold-drawing phenomena.
The Considxc3xa8re construction can be determined by the following equation:
"sgr"T="sgr"(1+xcex5)
where:
"sgr"T=true stress
"sgr"=engineering stress
xcex5=draw ratio=(Lxe2x88x92Li)/Li
where:
L=sample length under deformation
Li=initial sample length
The thermoformable compound must also exhibit acceptable elongation characteristics at elevated temperature. If the elongation is too low, the sheeting will tear when thermoformed. Thus, two particularly significant tensile properties are true ultimate tensile strength at 140xc2x0 C. and elongation to break at 140xc2x0 C. Under extreme draw conditions of some thermoforming applications, the Heck et al. compositions form holes leading to part failure.
Compositions having greater melt extensibility can be produced by lowering the level of peroxide used for rheology modification. However, lower peroxide levels result in lower melt strength and less tensile strength. Thus, there is a need to produce rheology-modified TPE compositions having an improved melt toughness. Further there is a need to enhance the high temperature tensile properties of such compositions for thermoforming applications.
Applicant has found that rheology modification by addition of at least one peroxide and at least one free radical coagent has a signicant effect on the melt toughness and high temperature tensile properties of blends of at least one elastomeric EAO polymer or EAO polymer blend and a polyolefin such as PP. The rheology modified compositions of this invention have melt toughness and high temperature tensile properties that are higher than corresponding compositions rheology modified by the addition of peroxides alone. As such, one aspect of this invention is a rheology-modified, substantially gel-free thermoplastic elastomer (TPE) composition comprising an EAO polymer or EAO polymer blend and a high melting polymer selected from the group consisting of polypropylene homopolymers and propylene/ethylene copolymers, wherein the composition is rheology modified by at least one peroxide and at least one free radical coagent and the rheology modified composition has a melt toughness of at least 600 centinewton millimeters per second (cNmm/s), a true ultimate tensile strength at 140xc2x0 C. of at least 3 mega-Pascals (MPa) and an elongation to break at 140xc2x0 C. of at least 400%. The TPE compositions may be compounded with conventional additives or process aids including, for example, fillers, stabilizers, dispersants, pigments and process oils. Compounds prepared from the rheology modified polymers of this invention retain their processing advantages over compounds prepared from the same polymers, but rheology modified by peroxide alone.
A second aspect of this invention is a process for preparing a rheology-modified, substantially gel-free TPE composition, the process comprising: a) adding at least one peroxide and at least one free radical coagent to a molten polymer blend that comprises an elastomeric ethylene/alpha-olefin polymer and a high melting polymer selected from the group consisting of polypropylene homopolymers and propylene/ethylene copolymers; and b) maintaining the polymer blend in a molten state while subjecting it to conditions of shear sufficient to disperse the peroxide and coagent throughout the molten polymer blend, effect rheology modification of the polymers and substantially preclude formation of insoluble polymer gels, sufficient rheology modification being measured by a melt toughness of at least 600 centinewton millimeters per second (cNmm/s), a true ultimate tensile strength at 140xc2x0 C. of at least 3 mega-Pascals (MPa) and an elongation to break at 140xc2x0 C. of at least 400%. The process optionally includes a step c) wherein the rheology modified polymer blend is converted to an article of manufacture, preferably without intermediate steps of recovering the rheology modified polymer blend as a solid and then converting the solid to a melt state sufficient for fabricating the article of manufacture. If desired, however, the process optionally includes the intermediate steps.
One variation of the second aspect is a process for preparing a rheology-modified, substantially gel-free TPE composition, the process comprising: a) adding at least one peroxide and at least one free radical coagent to at least one component of a polymer blend that comprises an elastomeric ethylene/alpha-olefin polymer and a high melting polymer selected from the group consisting of polypropylene homopolymers and propylene/ethylene copolymers; and b) converting the polymer blend to a molten polymer blend while subjecting the blend to conditions of shear sufficient to disperse the peroxide and coagent throughout the molten polymer blend, effect rheology modification of the polymers and substantially preclude formation of insoluble polymer gels, sufficient rheology modification being measured by a melt toughness of at least 600 centinewton millimeters per second (cNmm/s), a true ultimate tensile strength at 140xc2x0 C. of at least 3 mega-Pascals (MPa) and an elongation to break at 140xc2x0 C. of at least 400%. The process optionally includes a sequential step c) wherein the rheology modified polymer blend is converted to an article of manufacture, preferably without intermediate steps of recovering the rheology modified polymer blend as a solid and then converting the solid to a melt state sufficient for fabricating the article of manufacture. If desired, however, the process optionally includes the intermediate steps.
A second variation of the second aspect is a process for preparing a rheology-modified, substantially gel-free thermoplastic elastomer article of manufacture, the process comprising: a) adding at least one peroxide and at least one free radical coagent to a molten elastomeric ethylene/alpha-olefin polymer or elastomeric ethylene/alpha-olefin polymer blend to provide a rheology-modified ethylene/alpha-olefin polymer or ethylene/alpha-olefin polymer blend; b) adding to the rheology-modified polymer or polymer blend a high melting polymer selected from the group consisting of polypropylene homopolymers and propylene/ethylene copolymers to form a composite polymer blend; and c) converting the composite polymer blend into the article of manufacture, the article of manufacture having a melt toughness of at least 600 centinewton millimeters per second (cNmm/s), an true ultimate tensile strength at 140xc2x0 C. of at least 3 mega-Pascals (MPa) and an elongation to break at 140xc2x0 C. of at least 400%.
A third aspect of this invention is an article of manufacture having at least one component thereof fabricated from the TPE composition of the first aspect of the invention. The TPE compositions suitably include at least one additive selected from the group consisting of process oils, fillers and blowing agents. The compositions readily allow formation of articles of manufacture using apparatus for calendaring and/or thermoforming. In a related aspect, the TPE compositions of the first aspect may be blended with another polymer, preferably one of the polymers used to make the TPE composition, prior to fabrication of an article of manufacture. Such blending may occur by any of a variety of conventional techniques, one of which is dry blending of pellets of the TPE composition with pellets of another polymer.